Use infinity to signal conics instead of NaN

From reading the GLSL ES spec, behavior around NaN handling is not
well defined. Infinity and isinf(), on the other hand, seems to be
well defined.

Bug: skia:10419
Change-Id: Id465a89a0d921707021974b5a64ec88bbcce0c33
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/341076
Commit-Queue: Chris Dalton <csmartdalton@google.com>
Reviewed-by: Ethan Nicholas <ethannicholas@google.com>
3 files changed